Scorsese didn't care less about LaMotta. his narrow sensibilities were completely unsympathetic to the man ('uh, some sports guy') and he had to be coaxed into making the film by De Niro, whose pet project/ego-vehicle it was. of course, Scorsese's attitude to the piece became very self-satisfied and warm as he revelled in all the unexpected critical gushing and 'masterpiece' accolades heaped on it down the years (over-praise, imo) - as such, he's more than happy to talk about it now as if it were some great passion of his creative life, for the reason that it has earned him so much flattering kudos.
De Niro, meanwhile, captured some superficial sense of LaMotta, but was unable to channel Jake's soul (or, perhaps, even realize the depth of it to begin with). it was a macho-vanity performance.

i think it's a heavily flawed film in its unmindfully skewed reflection of Jake. but it looks pretty (here, Scorsese did invest some passion) and Moriarty is worth the price of admission.